Finding Instance URL in Addon

I have been trying to complete my first addon and it needs to know the URL that it it is being served on so that it can correctly set it’s CORS headers, as part of ingress. I might be missing the obvious, but how can I do this?

I have tried setting weburl in ‘config.yml’ and then calling ‘bashio::addon.webui’ but that returns blank. I have seen that there is a ‘get_URL()’ call as part of an integration, but I am not sure how that is exposed, if at all via APIs.

Any suggestions on where to look?